The present invention relates to the encoding of audio and video signals and in particular to producing segments of audio and video material that can be joined together on the fly.
It is typically the case that, when two video clips are required to be played one after the other, the decoder is required to reset itself to a starting state before decoding the second clip. During this reset, the user will be presented with the last frame of the first clip frozen on the screen, together with a mute of the accompanying audio. This discontinuity is intrusive to the user.
What is required is a seamless join in which the transition between the end of one clip and the start of the next is not noticeable to the decoder. This implies that from the user's point of view there is no perceptible change in the viewed frame rate and the audio continues uninterrupted. Applications for seamless video are numerous. An example from a CD-i perspective is the use of photo-real backgrounds for computer generated characters; an example use of this technique would be an animated character running in front of an MPEG coded video sequence. Another is a series of character-user interactions such as interactive movies where the viewer has the chance to influence development of the storyline by selecting from available alternative scenarios. Branch points along the path a user chooses to take through an interactive movie should appear seamless, otherwise the user will lose the suspension of disbelief normally associated with watching a movie.
A method for encoding segments of video such that re-initialisation of the decoder is not required is described in our UK Patent Application number 9424436.5 (PHB 33950) entitled "Video Editing Buffer Management" filed Dec. 2, 1994. The method uses targeting of decoder buffer levels to give consistency of decoder buffer occupancy for the end of each video clip and predictability for the start of each segment such as to allow successive clips to be joined directly without risking overflow or underflow in the decoder buffer.
The above method, whilst suitable for sequences of video frames, takes no account of the other information that will typically accompany it--for example an audio soundtrack. The video will generally be interleaved with other information into a single stream, referred to as the system stream, which constitutes the data that will be presented to a decoder/display device (for example the data stream read from a compact disc or delivered via a cable network).